1. Field of the Invention
The present invention relates to atomizing devices and to methods of making the same and, more particularly, to micromachined atomizing devices that produce small droplets with a narrow droplet size distribution and to methods of making the same.
2. Description of the Related Art
Liquid atomizing devices are used in various mechanisms, such as medical nebulizers and fuel injectors for combustion chambers. The performance of many of these mechanisms can be improved if the atomizing device provides a spray with very small droplets or a narrow size distribution. For example, small droplets improve the effectiveness of medical nebulizers because small droplets (e.g., between 2 and 5 micrometers) can be efficiently deposited deep into the lungs. Additionally, small droplets (e.g., less than 20 micrometers) improve the efficiency of combustion devices by causing faster vaporization of the fuel.
Conventional atomizing devices typically provide a spray having droplets within a wide range of sizes, including a small percentage of droplets that have a Sauter mean diameter smaller than 10 micrometers. Conventional atomizing devices have rarely been able to provide a spray having droplets limited to a small range of sizes and having a Sauter mean diameter smaller than 10 micrometers, without employing additional mechanisms such as high ultrasonic power or high-voltage electrostatic charging.
The failure of conventional atomizing devices to provide a small range and small droplets can be attributed to the manner in which these devices perform atomization. Conventional atomizing devices break bulk liquid into relatively large ligaments, break the ligaments into relatively large drops through atomization, and break the large drops into smaller droplets through secondary atomization. As the droplets become smaller than 100 micrometers, they become harder to break, and secondary atomization typically ceases, thus preventing most of the droplets from becoming as small as 10 micrometers. Also, since the bulk liquid is much larger than the desired droplet size and, therefore, must be broken down a number of times to become relatively small, the droplets ultimately formed by conventional devices will have a relatively wide size range.
Efforts have been made to decrease droplet size by increasing the amount of gas forced through the atomizing device. However, this results in a large gas-liquid mass ratio, which is undesirable for many applications because it requires a large gas pump, a large amount of gas, and a high gas velocity.
Another problem associated with conventional atomizing devices is that two devices, even of the same type, often will have different spray characteristics. These differing spray characteristics result from very minor variations in the structure of the atomizing device. With current manufacturing methods, these variations occur more frequently than is desired.